Self-calibrating ultrasonic thickness-measuring apparatus

ABSTRACT

Pulsed ultrasonic thickness-measuring apparatus utilizing three ultrasonic transducers equally spaced from the adjacent surface of an object whose thickness is to be measured. Two transducers function as conventional transmitter and receiver to direct pulses of ultrasonic energy into the object and receive energy reflected from the far surface of the object or from a defect within the object. The third transducer is a transceiver that directs pulses of ultrasonic energy onto the adjacent surface of the object and receives reflections therefrom. A reflection received by the third transducers commences a time measuring operation indicating thickness of the object, or location of defect, as measured by the first two transducers. Use of a third transducer eliminates from timing operation the transit time of ultrasonic energy from transmitter transducer to adjacent surface, and from said surface to receiver transducer, thereby providing a measure only of thickness of object.

United States Patent [72] Inventor Hillel Weinbaum OTHER REFERENCESHouston Curtis-Wright, K-H Two Channel Flaw Alarm, Publication [211 PP780,663 ofCurtis-WrightCorp., cs 106-000 received June 1961. [22] FiledDec. 3, 1968 [45] patented 17 1971 Primary Examiner Richard C. Queisser[73] Assignee AMF Incorporated Assistant Examiner-John P. BeauchampAttarneysGeorge W. Price and John H. Gallagher ABSTRACT: Pulsedultrasonic thickness-measuring ap- [54] SELECALIBRATING ULTRASONICTHICKNESS paratus utilizing three ultrasonic transducers equally spacedMEASURING APPARATUS from the ad acent surface of an ob ect whosethickness is to be 3 claimsznrawing Figs measured. Two transducersfunction as conventional transmitter and receiver to direct pulses ofultrasonic energy into [LS- the bj t a d re eive energy reflected fromthe far urface of 73/67-9 the object or from a defect within the object.The third trans- [5l I III!- Cl ..G01ll 29/04, ducer i a t an eiver thatdirect pulses of ultrasonic energy Golb 17/02 onto the adjacent surfaceof the object and receives reflec- [50] Field Of Search 73/67.7- {ionsth f m A flection received by the third transducers 67-9 commences atime measuring operation indicating thickness of the ob'ect, or locationof defect, as measured b the first [56] References cued two transducers. Use of a third transducer eliminites from UNITED STATES PATENTStiming operation the transit time of ultrasonic energy from 3,164,0071/1965 Stebbins et al 73/67.9 transmitter transducer to adjacentsurface, and from said sur- 3,228,232 1/1966 Proctor 73/67.7 face toreceiver transducer, thereby providing a measure only 3,485,087 12/1969Brech 73/67.7 of thickness of object.

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FIG. 2

INVENTOR.

HILLEL WEINBAUM BY M ATTORNEY SELF-CALIBRATING ULTRASONIC THICKNESS-MEASURING APPARATUS BACKGROUND OF THE INVENTION Many different types ofultrasonic devices are known for measuring the wall thickness of objectssuch as pipes whose two surfaces are not readily accessible to theequipment which is located proximate one surface. This type of equipmentalso commonly is used to determine the presence and location ofstructural defects in the wall or body of the object being inspected. Acompact and reliable instrument of this type is disclosed and claimed inU.S. Pat. application Ser. No. 598,774, entitled Ultrasonic ThicknessMeasuring Apparatus, filed Dec. 2, 1966 by the present applicant. Thisapparatus utilizes a transmitter transducer to direct pulses ofultrasonic energy into the wall of the object, a pipe for example, beingtested and employs a receiver transducer to receive pulses reflectedfrom the far surface of the wall. In that application, timingmeasurements for determining the thickness of the wall are made withreference to the occurrence of a clock' pulse from a clock pulse sourcewhich also actuates the transmitter transducer. The actual timingoperation is commenced a fixed time interval after the occurrence of theclock pulse to account for the transit time of the ultrasonic energyfrom the transmitter transducer to the adjacent surface of the objectand from that surface to the receiver transducer, the transducers beingslightly spaced from the adjacent surface. This technique is accurate ifthe transducers always are spaced the same distance from the adjacentsurface of the object being inspected. However, when the objects beinginspected are pipes and tubing used in the oil, gas and petrochemicalindustries, and particularly if the pipes are used pipes, the surfacesoften have scale, rust, and other foreign matter thereon and thetransducers are not always positioned the same distance from the actualsurface of the pipe. Additionally a coupling medium such as a fluid or agrease often is used between the transducers and the surface of theobject being tested to provide an improved impedance match between thetransducers and the object, thereby to improve the coupling of theultrasonic energy into and out of the object. Obviously, the thicknessof this coupling medium cannot be maintained constant, particularly whenoperating the equipment in the field and when the transducers are withina hand-held mounting head which is placed on the pipe surface by anoperator. It is believed obvious that the system described in applicantsabove-identified pending application is susceptible to some inaccuracysince it has a fixed although manually adjustable, time delay built intoits timing systemwhile the actual transit times of the ultrasonic energyto and from the adjacent surface of theobjectbeing inspected is subjectto variation.

SUMMARY OF THE INVENTION In the present invention the possibleinaccuracies inherent in the above-described system are substantiallyeliminated by eliminating the fixed time delay feature. In thisinvention, a third ultrasonic transducer, operating as a transceiver ispulsed by the same clock pulses that pulse the transmitter transceiver,and receives energy reflected directly from the adjacent surface of theobject such as a pipe whose wall thickness is being measured. Thereceived pulse from this third transducer commences the time measuringoperation at such a time as to substantially eliminate from the finaltime measurement, which indicates pipe wall thickness, the transit timeof the ultrasonic energy from the transmitter transducer to the adjacentsurface and from that surface to the receiver transducer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagramillustrating the u]- trasonic thickness-measuring system of thisinvention, and;

FIG. 2 is a series of waveforms referred to in describing the operationof the system in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring in detail to thedrawings, the object 5 whose thickness is to be measured will be assumedin this description to be the wall of a steel pipe, although it couldjust as well be a metal bar, rod, plate or some other object. Thesurface 6 of the wall would be the outside surface of the pipe which isaccessible for measurement, and surface 7 would be the inside surfacewhich is inaccessible throughout most of the length of the pipe. In thefollowing description, surfaces 6 and 7 will be referred to as adjacentand far surfaces, respectively. Furthermore, it will be assumed that itis desired only to measure the wall thickness of the object 5. It isobvious that the presence and location of internal structural defects inthe wall also may be determined with the apparatus of this invention.

A clock pulse source 10 produces regularly recurring electrical pulses,FIG. 2a, which are coupled over lead 11 to a pulser, or modulator, l3.Pulser 13 operates in response to input clock pulses to producecorresponding output pulses, FIG. 2b,of greater amplitude which arecoupled over lead 15 to a transmitter ultrasonic transducer 17 that islocated within a mounting head 18 which in practice might be a smallcylindrical member that is hand-held by the operator of the apparatus.Also located within mounting head 18 is a receiver ultrasonic transducer19, and a third ultrasonic transducer 21 which functions as atransceiver transducer, as will be explained. Preferably, all threetransducers are positioned within mounting head 18 so that their facesclosest surface 6 are substantially equally spaced therefrom whenmounting head 18 is properly positioned on surface 6.

Transmitter and receiver transducers l7 and 19 have their respectivetransmitting and receiving surfaces slightly inclined with respect tothe adjacent surface 6 of object 5. Transducers l7 and 19 may be knowntypes of piezoelectric crystals that are constructed and arranged totransmit and receive, respectively, a relatively narrow directed beam ofultrasonic energy that propagates along the illustrated path comprisedof path segments 25 and 27. In accordance with the present invention, itis desired that there be substantially no reflection of transmittedultrasonic energy from the adjacent surface 6 directly back to receivertransducer 19. Design techniques for substantially assuring this type ofoperation are well known to those skilled in the art, and as one examplein which the faces of transducers 17 and 19 both were less than 0.070inch from adjacent surface 6, an angle of approximately between thosefaces produced the desired type of operation.

Electrical pulses from pulser 13, FIG. 2b,also are coupled over lead 30to pulse or modulate transceive transducer 21 in time coincidence withthe pulsing of transmitter transducer 17. Transducer 21 may be similarin construction to transducers l7 arid 19 but it is arranged with itsface substantially parallel to adjacent surface 6 so that the beam pathof ultrasonic energy transmitted therefrom is substantially normal tosurface 6, whereby ultrasonic energy will be reflected from surface 6directly back to transducer 21 which then functions as a receivertransducer to produce corresponding electrical signals on lead 32.

After transmitter transducer 17 is pulsed by pulse 34 of FIG. 2b, forexample, the corresponding pulse of ultrasonic energy follows pathsegment 25 to the far surface 7 of object 5, is reflected therefrom, andfollows path segment 27 to receive transducer 19 which responds theretoto produce the electrical pulse 36, FIG. 2c, which is amplified inamplifier 38 and coupled to single pulse generator 40.

It is seen in FIG. 2c that additional pulses appear following pulse-36and that those following pulses successively decrease in magnitude.Those following pulses arise from successive reflections of ultrasonicenergy which is trapped within object 5 between adjacent and farsurfaces 6 and 7, as is well understood by those skilled in the art.Only pulse 36 out of the group is desired for subsequent operation ofthe system. Succeeding pulses are eliminated by single pulse generator40 which produces a single pulse output, FIG. 2d, in response to pulse36 of FIG. 20. Pulse generator 40 is completely nonresponsive to thesucceeding pulses of a group of pulses illustrated in FIG. 20. Singlepulse generator 40 may take any number of difierent forms, but onesuitable circuit arrangement is comprised of two monostablemultivibrators connected in tandem. The first one has a long unstablestate which is longer than the duration of one group of pulses of FIG.20. Consequently, pulse 36 would cause the multivibrator to change fromits stable to its unstable state but it would not be further affected bythe succeeding pulses of that group. The second multivibrator has ashort time constant so as to produce the desired output pulse 41 of FIG.2d when triggered by the initial change of state of the firstmultivibrator. Devices operating in this manner are commerciallyavailable from Fairchild Semiconductor Corp., under the numericaldesignation 9601.

The single pulse output of pulse generator 40, FIG. 2d, is coupled fromlead 45 to the reset input of bistable multivibrator 46. The set inputto multivibrator 46 now will be described.

After transceive transducer 21 is pulsed simultaneously with transmittransducer 17 by pulse 34 of FIG. 2b, it receives directly back theultrasonic energy reflected from adjacent surface 6. The correspondingtransducer electrical signals appear on lead 32 as the waveform of FIG.2e, and after amplification, are coupled to one input terminal of ANDgate 48. The other input to AND gate 48 is the waveform of FIG. 2fproduced by monostable multivibrator 50 which is triggered by the clockpulse of FIG. 2a from clock source 10. The negative pulse 51 of FIG. 2f,which commences at the occurrence of the clock pulse, acts as an inhibitpulse to keep AND gate 48 closed until after the clock pulse, and anyassociated transients, are terminated. Waveform 2f then rises to itsmore positive level and conditions AND gate 48 to pass the pulse 53 online 32 which is connected to transceive transducer 21. The output ofAND gate 48 is the pulse 54 of FIG. 2g which appears on lead 55 andwhich is coupled to the set input terminal of bistable multivibrator 46.

It is desired that only the pulse 53 of FIG. 2e be passed by AND gate48, and if necessary, a single pulse generator similar to pulsegenerator 40 may be included in the connection between transducer 21 andAND gate 48.

Pulse 54, FIG. 23, when coupled to the set input of bistablemultivibrator 46 triggers it to its second stable state, FIG. 2h, andthe first pulse received thereafter on its reset input terminal, pulse41, FIG. 2d, resets it to its first stable state, whereby the output ofbistable multivibrator 46 is the timing pulse 58 ofFIG. 2h.

The time duration of timing pulse 58 is substantially equal to thetransit time of an ultrasonic energy pulse from transducer 17 as itpropagates from adjacent surface 6 to far surface 7 along path segment25, and from far surface 7 back to adjacent surface 6 along path segment27. Consequently, the duration of timing pulse 58 is a directmeasurement, in time, of the thickness of member 5. This is true becausetiming pulse 58 does not commence until after the ultrasonic energy fromtransceiver 21 has propagated to and from the adjacent surface 6 ofobject 5. That propagation time is substantially equal to the transittime of ultrasonic energy from transmit transducer 17 to surface 6 alongpath segment 25, and from surface 6 to receive transducer 19 along pathsegment 27. Therefore, the propagation time that transpires while theultrasonic energy is outside object is not included in the timemeasuring interval represented by timing-pulse 58.

To provide a suitable readout of timing pulse 58, the output of bistablemultivibrator 46 is coupled to the input of ramp generator and peakdetector circuit 60 which is well known circuitry which, in the firstinstance, functions in response to timing pulse 58 to produce a linearlyrising output signal which commences upon receipt of the leading edge ofpulse 58 and terminates its linear rise at the occurrence of thetrailing edge of pulse 58, as illustrated in FIG. 2i. The peak detectorportion of the circuit will maintain the peak voltage of the linear riseof the ramp-type output of the ramp generator.

Therefore, the amplitude 62 is a measure of the thickness of object 5.Suitable means (not illustrated) are provided to reset the rampgenerator and peak detector circuit 60 when required. This may beaccomplished by suitable circuitry operating in response to the outputof AND gate 48, as by the next pulse 71 of FIG. 2g, for example. Resetcircuits are commonly used in the art and their construction as requiredherein is well within the skill of one familiar with the art.

The output of ramp generator and peak detector 60 is coupled to asuitable thickness indicating instrument such as a meter 75 which iscalibrated to indicate thickness in units of linear measurement. Thetiming relationships represented in FIG. 2 are not to actual scale, forpurpose of convenience of illustration, and the ramp portion of thewaveform of FIG. 21' will in practice represent but a short timeinterval in the time interval between clock pulses of FIG. 2a, and willtherefore not adversely affect the meter reading.

In the inspection of pipe it is desirable to have an immediateindication of whether or not the measured wall thickness is withinacceptable limits. Circuitry that will automatically provide thisindication is shown in the bottom portion of FIG. 1. This circuitrycauses green light to light when the measured wall thickness is withinacceptable limits, causes red light 81 to light when the measured wallthickness is below the minimum acceptable limit, and causes white light82 to light when the measured wall thickness is greater than the maximumacceptable limit.

The output of AND gate 48, FIG. 2g, which will occur prior to thereceipt by receive transducer 19 of a pulse reflected from far surface7, is coupled over lead 84 to the input of minimum monostablemultivibrator 85. Upon the occurrence of the leading edge of pulse 54,FIG. 2g, minimum monostable multivibrator 85 is triggered to itsunstable state, FIG. 2j, and remains in that state for a time periodthat terminates at the time a reflected pulse would be received byreceive transducer 19 if the pipe wall were of minimum acceptablethickness. The output ofmultivibrator 85, FIG. 2j, is coupled to oneinput terminal of AND gate 87 and the output from single pulse generator40, FIG. 2d, is coupled to the other input terminal of AND gate 87. If apulser from pulse generator 40 appears at AND gate 87 coincidentallywith the pulse 88 of FIG. 2j, it will pass through AND gate 87 and willcause red light 81 to light, thus indicating that the measured wallthickness is below the minimum acceptable thickness.

The output of AND gate 48 also is coupled over lead 84 to a delaymonostable multivibrator 90 which is triggered by the leading edge ofpulse 54, FIG. 2g, to its unstable state, FIG. 2k, which persists for atime sufficient to permit it to return to its stable state when receivetransducer 19 would receive a reflected pulse from the far surface of anobject whose thickness was the maximum acceptable thickness, thiscorresponding to the trailing edge of pulse 92, FIG. 2k. The output ofdelay multivibrator 90 is coupled to the input of maximum monostablemultivibrator 94 which responds to the trailing edge of pulse 92 tochange to its unstable state, FIG. 2m, for a time interval representedby pulse 95, this time interval occurring beyond the time that areflection would be received by transducer 19 from the far surface of apipe whose wall thickness is within limits. The output of maximummonostable multivibrator 94 is coupled to one input of AND gate 97 andenables that gate to pass a pulse from single pulse generator 40 onlyduring the occurrence of pulse 95. Any such pulse passed by AND gate 97will light white light 82 and indicate that the wall thickness is toogreat.

Bistable multivibrator 100 is triggered to its second stable state, FIG.2n, by the trailing edge of pulse 88, FIG. 2j, coupled to its set inputterminal over lead 102 connected to the output of minimum multivibrator85. Bistable multivibrator 100 is reset to its first stable state by theleading edge of pulse 95, FIG. 2m, coupled to its reset terminal overlead 104 from the output of maximum multivibrator 94. Because thetrailing edge of pulse 88 defines the minimum acceptable wall thicknessand the leading edge of pulse defines the maximum acceptable wallthickness, the pulse 105 of FIG. 2n occurs only during the time thatreceive transducer 19 would receive a reflection from a far surface 7 ofan object whose thickness is within acceptable limits.

The output of bistable multivibrator 100 is coupled to AND gate 107 andwill enable that gate to pass pulses when the thickness of object 5 iswithin limits, thereby lighting green light 80.

Other types of circuitry can be 'built to perform the same toleranceindications as just described.

From the above description it is believed to be apparent that theresulting thickness measurement is extremely accurate because thecircuitry, in effect, measures time only during the interval that theultrasonic energy is propagating within the object. Consequently, noerror can arise in the measurement because of variations orimprecisionin the spacing of the transducers from the adjacent surface of theobject being inspected.

What I claim is:

l. Ultrasonic-measuring apparatus for inspecting an object having anadjacent surface and a far surface which reflects ultrasonic energy,comprising first and second ultrasonic transducers disposed proximatesaid adjacent surface and each angularly oriented thereto fortransmitting ultrasonic energy from the first transducer and throughsaid adjacent surface with substantially no reflection from the adjacentsurface back to the second transducer when the transmitted ultrasonicenergy is obliquely oriented at a desired angle to said adjacentsurface, the second transducer receiving ultrasonic energy reflectedfrom said far surface or reflected from a defeet between said surfacesof the object and propagated back through said object and out of theadjacent surface,

a third ultrasonic transducer disposed proximate said adjacent surfaceand proximate said first and second transducers for transmittingultrasonic energy substantially normally onto the adjacent surface toprovide ultrasonic energy reflected directly from the adjacent surface,said directly reflected energy being received by the third transducerbut not by the second transducer,

said first and third transducers being substantially equally spaced fromthe adjacent surface,

means for energizing said g, and third transducers in substantial timecoincidence, whereby they transmit ultrasonic energy in substantial timecoincidence,

an electrical bistable device which may be transferred between first andsecond stable states by respective signals applied thereto, 7

means for coupling said third ultrasonic transducer to said bistabledevice to transfer said device to its first stable state upon thereceipt by said third transducer of a direct reflection of itsultrasonic energy from the adjacent surface,

means for coupling said second ultrasonic transducer to said bistabledevice to transfer said device to its second stable state upon thereceipt by said second transducer of a reflection of ultrasonic energyfrom the far surface of the object or from a defect between the surfacesof the object, andmeans for providing an indication of the time intervalthe bistable device is in its first stable state.

2. The combination claimed in claim 1, wherein said means for energizingsaid first and third transducers substantially in time coincidenceincludes a common source of electrical signals.

3. The combination claimed in claim 2, wherein said source is a sourceof recurring pulses.

1. Ultrasonic-measuring apparatus for inspecting an object having anadjAcent surface and a far surface which reflects ultrasonic energy,comprising first and second ultrasonic transducers disposed proximatesaid adjacent surface and each angularly oriented thereto fortransmitting ultrasonic energy from the first transducer and throughsaid adjacent surface with substantially no reflection from the adjacentsurface back to the second transducer when the transmitted ultrasonicenergy is obliquely oriented at a desired angle to said adjacentsurface, the second transducer receiving ultrasonic energy reflectedfrom said far surface or reflected from a defect between said surfacesof the object and propagated back through said object and out of theadjacent surface, a third ultrasonic transducer disposed proximate saidadjacent surface and proximate said first and second transducers fortransmitting ultrasonic energy substantially normally onto the adjacentsurface to provide ultrasonic energy reflected directly from theadjacent surface, said directly reflected energy being received by thethird transducer but not by the second transducer, said first and thirdtransducers being substantially equally spaced from the adjacentsurface, means for energizing said g, and third transducers insubstantial time coincidence, whereby they transmit ultrasonic energy insubstantial time coincidence, an electrical bistable device which may betransferred between first and second stable states by respective signalsapplied thereto, means for coupling said third ultrasonic transducer tosaid bistable device to transfer said device to its first stable stateupon the receipt by said third transducer of a direct reflection of itsultrasonic energy from the adjacent surface, means for coupling saidsecond ultrasonic transducer to said bistable device to transfer saiddevice to its second stable state upon the receipt by said secondtransducer of a reflection of ultrasonic energy from the far surface ofthe object or from a defect between the surfaces of the object, andmeans for providing an indication of the time interval the bistabledevice is in its first stable state.
 2. The combination claimed in claim1, wherein said means for energizing said first and third transducerssubstantially in time coincidence includes a common source of electricalsignals.
 3. The combination claimed in claim 2, wherein said source is asource of recurring pulses.